Turbo stall is usually down to air goung back through the turbo when you let off the throttle, this is usually caused by a stiff dump valve ir no dump valve. If you can see it in the engine bay have someone rev the engine whilst you check it works properly, if it's failing you cN have them rebuilt but maybe best to just buy a new one, maybe a different company, I know how to make them stiffer but not looser.